In general, when a rotor is attached to a nacelle of a wind turbine generator, a plurality of, for example, three, rotor blades are attached to the rotor head on the ground to complete the rotor. Then, the horizontally disposed rotor is hoisted by a heavy machine and the like and attached to the nacelle on the top of the tower.
When the rotor blades are attached to the rotor head on the ground, the rotor is disposed such that the plane of rotation is substantially horizontal. On the other hand, when the rotor is attached to the nacelle, the rotor is raised such that the plane of rotation is substantially perpendicular (vertical).
Conventionally, a single large heavy machine holds the rotor head at a position offset from the center of gravity of the rotor, and a single small heavy machine holds the tip of one of the rotor blades.
First, the large heavy machine and the small heavy machine cooperatively hoist the rotor, which maintains an orientation in which the plane of rotation remains substantially horizontal, from the ground into the upward. When the rotor is hoisted to a position where the rotor blade does not interfere with the ground even if the rotor is vertically raised, the small heavy machine stops hoisting and the large heavy machine continues hoisting. Because the large heavy machine holds the rotor head at a position offset from the center of gravity of the rotor, the rotor is vertically raised because of its weight.
Besides the above-described method, a method in which the rotor head is attached to the nacelle first, and the rotor blades are hoisted and sequentially attached to the rotor head has been proposed (for example, Patent Document 1).
This method provides an advantage in that a place for assembling the rotor does not need to be secured on the ground. The above-mentioned advantage is significant especially when constructing a wind turbine generating unit in a place like a mountainous area, where securing flat land is difficult.    Patent Document 1: Japanese Translation of PCT International Application, Publication No. 2005-531709